1. Field of the Invention
The present invention relates to a pre-plasticizing injection molding machine equipped separately with a plasticizing part and an injection part, and more particularly, to the shape of an injection plunger in the injection part.
2. Description of the Prior Art
A screw pre-plasticizing injection molding machine is, in general, provided with a plasticizing part having a screw, and an injection part having an injection plunger. Molten resin plasticized in the plasticizing part is supplied to an injection chamber of the injection part by way of a path and then is injected into the cavity of a metallic mold through a nozzle by the injection plunger. Conventionally, there is provided a ball check valve in the path to prevent resin in the injection chamber from flowing back to the plasticizing part at the time of injection by the injection plunger.
Since the pre-plasticizing injection molding machine is equipped with an independent plasticizing part, it has a higher plasticizing capacity than an in-line injection molding machine. In particular, the pre-plasticizing injection molding machine works effectively in high-cycle molding operations, although molten resin may be retained because of the ball check valve. There is something left to be desired when handling such matters as they easily are subject to thermal cracking.
Recently, there has been proposed a pre-plasticizing injection molding machine with no ball check valve to eliminate the above problem. This injection molding machine is equipped with a rotary motor to turn a mixing screw and an actuator to axially move the screw. When plasticizing and weighing, the screw retreats to produce an opening between the screw front end and the path. Molten resin is supplied to the injection chamber of the injection part. When injecting, the screw advances and a front end thereof closes the path opening, and molten resin thereby is prevented from flowing back to the plasticizing chamber. Such injection molding machine which keeps molten resin from flowing back by an advancing/retreating screw does not experience the problem of molten resin being retained because of a ball check valve. In addition, in enables accurate weighing as well as well stabilized plasticizing, and has characteristics of providing excellent molding condition reproducibility, rising, and stability. With this injection molding machine, CAD, CAM and CAI (computer aided injection molding) which associate a metallic mold manufacturing machine with an injection molding machine also are feasible.
As shown in FIG. 11, such prior injection molding machine has a hole 27 open to the center of a cylinder front wall 23a of injection chamber 17 and leading to an injection nozzle. A hole 30b opens to the front wall adjacent a peripheral wall 15a of the cylinder and leads to a plasticizing part through a path 30. An injection plunger 16' has a flat front face 16'f and a head or piston 16'a moving slidably in contact with the cylinder peripheral wall 15a and a shank or rod 16'b much smaller in diameter than the head 16'a.
At a pressure holding process which follows an injection process, there is produced a clearance of a few millimeters as a minimum amount of cushioning between the front face 16'f of the injection plunger 16' and the front wall 23a of the cylinder, providing a specified holding pressure on the molten resin. Thus, no sink mark is generated in the molding due to cooled resin in the cavity. After the holding process, weighing starts over when molten resin flows into the injection chamber 17 through the path opening 30b. The flow of resin from the path opening 30b pressures mainly the head 16'a as the front wall 23a and the head front 16'f are flat and parallel. As a result, no resin flows into a closed space opposite to the opening of the nozzle 27, as shown in FIG. 12. The flow of resin is diverted in the direction of arrows B, pushing the injection plunger 16'. Resin not washed by a new flow of resin may be retained thereby in the closed space A in contact with and on the front wall 23a. The retained resin may become thermally deteriorated, thus affecting molding adversely. In addition, rising may be inferior at the time of a color change.
Shank 16'b maintains friction on the head 16'a with the cylinder peripheral wall 15a within a specified range. Molten resin leaking from between the head 16'a and the cylinder peripheral wall 15a pools in a wide space D between the shank 16'b and the cylinder peripheral wall 15a, and is caused to form into a weir because of reciprocation of the injection plunger. Carburizing occurs due to heat generated in the cylinder, such that the leaked resin grows into a hard mass that cannot be removed easily. In addition, such a hard mass may force the injection plunger to move in an inclined manner which hinders smooth reciprocation.
If the injection plunger has the same diameter as the head, the contact area of the injection plunger with the cylinder peripheral wall will be wider, and the injection plunger will reciprocate in a heavier manner. The plunger will also suffer uneven wear and become grooved more readily, reducing its durability.
Such a problem occurs when a pre-plasticizing injection molding machine enables such problem-solving, accurate, efficient injection molting operation by using an advancing/retreating screw to prevent molten resin from flowing back.
The present invention aims therefore at providing a pre-plasticizing injection molding machine capable of solving the above problem by washing a closed space in the injection chamber with a new flow of molten resin from a path opening to prevent molten resin from being retained.